Congestive heart failure (CHF) is associated with impaired myocardial cell function. Alterations in the content and isoform distribution of contractile proteins and key proteins involved in calcium handling have been found in CHF. Thus, decreased force development in CHF may result from altered calcium handling or from reduced contractile protein function. Conflicting results regarding calcium handling in CHF have been obtained, which may be due to uncertainties with the methods used to measure intracellular calcium. Likewise, whether contractile protein function is impaired in CHF is also uncertain because sarcomere length was not measured and controlled. Thus, the relationship between alterations in protein synthesis and impaired myocardial cell function in CHF is still uncertain. Accurate knowledge regarding the cellular processes that participate in the development of CHF is critical to the development of innovative strategies aimed to combat CHF. The overall goal of the present research proposal is to determine the mechanism of reduced myocardial force development in an experimental animal model of CHF. The experimental animal model that we will use is CHF in rats, induced by left ventricular myocardial infarction. The right ventricles of these small rodents can provide trabeculae that are sufficiently thin and homogenous to allow i) state of art mechanical studies at the level of the sarcomere, and ii) accurate calibrated measurements of intracellular calcium. Preliminary data, using this model, indicate depressed function of isolated right ventricular trabeculae at end-stage CHF. Specifically we will test the hypothesis that reduced myocardial force development during the development of CHF is caused by: 1) Reduced intracellular calcium concentration during systole. Intracellular calcium will be measured directly in trabeculae by fluorescence calcium probe. To obtain unambiguous data, in-vivo calibration will be obtained by using the free salt of Indo-1, introduced into the cytolol by a recently developed technique. 2) Reduced maximum force development or calcium responsiveness of the contractile apparatus. Force development will be measured in permeabilized trabeculae as function of free calcium. Laser diffraction techniques will be used to accurately measure and control sarcomere length such that accurate and unambiguous data are obtained.